README: added Travis CI "badge" (status image)

The original repo (https://travis-ci.org/BruceSherwood/vpython-wx) is linked instead of my fork (currently set up in Travis) thus there is an external pending change.
README: set build status for specific branches

For "master" and "stable" (which doesn't exist yet but it probably will)
Merge pull request #21 from glarrain/ci

CI: added build status "badge" to the README
Update .travis.yml: added commands to install APT packages
Merge pull request #23 from glarrain/installation

setup: fixed and improved requirements declaration
Remove junk files
Delete .pydevproject

Delete junk file.
Merge pull request #22 from glarrain/ci

Update .travis.yml: added commands to install APT packages
Update .travis.yml: added command to install APT package "python-dev"

This build https://travis-ci.org/BruceSherwood/vpython-wx/jobs/11764893 failed because of this, and maybe something else
Update .travis.yml: added command to install APT package "libboost-python-dev"

Build still fails
Update .travis.yml: added command to install APT package "libboost-signals-dev"

Build still fails but is going further
Merge pull request #25 from glarrain/patch-1

Update .travis.yml: added commands to install APT packages